For example, Patent Document 1 shows an engine performing compression ignition combustion when the engine operates in a predetermined low-speed low-load range. In the range for performing the compression ignition combustion, this engine provides a period of closing both of intake and exhaust valves (i.e., a negative overlap period) across an exhaust top dead center, thereby introducing part of exhaust gas into a cylinder (precisely, the part of exhaust gas is confined in the cylinder). Patent Document 1 also teaches controlling the closing time of the exhaust valve in accordance with the level of the engine load such that the lower the engine load is the more internal EGR gas is introduced into the cylinder. Since internal EGR gas has a relatively high temperature, the more internal EGR gas is introduced into the cylinder, the higher the compression start temperature and the compression end temperature become. This increases ignitionability in compression ignition and stability in the compression ignition combustion at a low engine load.
Patent Document 2 shows an engine performing compression ignition combustion of an air-fuel mixture in a cylinder when the engine operates in a predetermined low-speed low-load operation range. In the operation range for performing the compression ignition combustion, the engine introduces into the cylinder, ozone together with fresh air. Patent Document 2 also teaches that the lower the engine load is, the more the ozone is introduced into the cylinder, thereby increasing ignitionability in compression ignition and combustion stability.